Well, if you're too lazy to read the whole AAIB initial report, here's the most salient part of it:
Initial indications from the interviews and Flight Recorder analyses show the flight and approach to have progressed normally until the aircraft was established on late finals for Runway 27L. At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond. The aircraft speed reduced and the aircraft descended onto the grass short of the paved runway surface.
So can we please put all speculations about this being another Kegworth to rest, pleeeeeease.
Also anyone who suggests that approach should be stabilized by 1000 AGL, with power and speed IAW final approach settings, or that you need engine out of approach idle to fly 3° glidepath, has not actually flown into Heathrow, or for that matter any other major European airport for at least last seven years (and probably more). Due to noise abaitment and traffic sequencing requirements, approaches at LHR are flown with intermediate flaps, gear retracted and 160 kt untill four DME. At 4 miles out, gear goes down, flaps goes towards landing setting and as you need to decelarete towards Vapp, engines go to flight idle. Depending on your weight and prevailing wind, you'll be fully established (speed, power, config) between 900 and 500 ft AGL and it seems that this was the point where first signs of trouble became apparent. Less than a minute later, airplane hit the dirt. Congrats to the crew, whatever they did, they've kept the airplane upright untill the ground contact and that's not a small thing.